Electrostatic discharge (ESD) is a continuing problem in the design, manufacture, and utilization of integrated circuits (ICs). A major source of ESD exposure to ICs is from the human body (described by the “Human Body Model”, HBM). In this situation, a packaged IC acquires a charge when it is held by a human who is electrostatically charged (e.g. from walking across a carpet). A charge of about 0.6 uC may be induced on a body capacitance of 100 pF, for example, leading to an electrostatic potential of 4 kV or more and discharge peak currents of several amperes to the IC for approximately 100 ns. A second source of ESD exposure is from charged metallic objects (described by the “Machine Model”, MM), which is characterized by a greater capacitance, lower internal resistance and transients that have significantly faster rise times and higher peak current levels than a HBM ESD source. A third source of ESD exposure is due to the discharge of stored charge on the integrated circuit itself (described by the “Charged Device Model”, CDM), to ground with rise times of less than 500 ps. The current flow during CDM is in the opposite direction than from the HBM and MM ESD sources. Thus, an ESD device must provide protection from discharges both to and from the IC.
During an ESD event, ESD current is typically discharged between one or more of the IC pins and another object such as a human body, a metal object, or ground. When ESD current flows through vulnerable circuitry in the IC, the circuitry may be destroyed. Many conventional ESD protection techniques employ peripheral circuits to carry the ESD currents from the pins of the IC to ground by providing a low impedance path that bypasses more vulnerable circuits in the IC chip. In this way the ESD currents flow through the protection circuitry rather than through the more vulnerable circuits in the chip.